The present invention relates to the field of semiconductor wafer processing and, more particularly, to monitoring plasma etching.
Plasma etching is a well known dry etching process in the art of semiconductor fabrication. It is also known that the endpoint of a plasma etching process may be determined by monitoring the level of emission of selected byproducts of the etching reaction. For example, an endpoint detector may be used for detecting byproducts of the etched layer in the exhaust stream of the etching chamber. The etching process is stopped when no more byproduct is detected.
Endpoint detection may also rely on light emitted by the plasma during the etching reaction. This light includes emissions at wavelengths representative of specific substances present in the plasma. Therefore, the level of a reaction byproduct may be monitored through measurement of emissions at the byproduct""s particular emission wavelength. By monitoring the level of a byproduct which originates from the layer being removed by the etching, the endpoint of the process is indicated by a sharp drop in the level of emitted reaction product. For example, in a plasma etching process wherein a layer of oxide is being removed, carbon monoxide (CO) is a typical byproduct released into the plasma. CO emits light at a wavelength of 483 nm. Therefore, when the oxide layer is fully removed, the CO emission decreases indicating the endpoint of the etch process.
In making such emissions measurements, however, it is expected that etching of the oxide layer would appear to proceed at a fairly constant rate. Accordingly, CO emissions would also be expected to be fairly constant. Contrary to the expected result, however, in practice measurement of CO emissions displays a modulation having a period about equal to the time needed for etching a thickness of about half the wavelength used to detect the CO endpoint. This effect has been noted when measuring this byproduct at various wavelengths, so that the effect is independent of the wavelength. Modulation in the measured emission level results in a signal trace having peaks and valleys, making endpoint determination more difficult.
Experimental evidence indicates that the unexpected modulation in emissions measurements is due to wavelength interference produced by reflection of light from at least two levels on the wafer. Photons from the plasma are partially reflected and transmitted at the wafer surface. Photons reflected at the wafer surface produce a first reflection. The transmitted photons are then reflected at the silicon interface, forming a second reflection. The difference in the length of the optical paths for the first and second reflections produces an interference between the two reflections. When the extra distance traveled by the second reflection is an even multiple of one fourth of the wavelength, the interference is constructive. When the extra distance of travel is an odd multiple, the interference is destructive. This interference effect explains the inconsistent behavior of the signals observed.
The interference makes endpoint determination more difficult because an apparent drop in the emission being measured may be an artifact due to the modulation effect, rather than a true decrease indicating the endpoint of the etching process. This uncertainty causes an imprecise endpoint determination and contributes to reduced accuracy and quality control problems in the etching process.
In view of the foregoing background, it is therefore an object of the invention to provide an apparatus and method for increasing the precision of endpoint determinations in a plasma etching process for semiconductor wafer fabrication.
It is another object of the present invention to provide etch endpoint determinations which are consistent and reproducible as required in a semiconductor production environment.
These and other objects, features and advantages in accordance with the present invention are provided by a semiconductor processing apparatus which eliminates this interference by analyzing the plasma while excluding reflections from the semiconductor wafer. The apparatus includes a chamber having a window. The chamber has a substrate holder within for holding a semiconductor substrate. The chamber, which is a plasma etching chamber, also includes a plasma generator for generating a plasma within the chamber. An optical fiber having opposing first and second ends, is positioned adjacent the window such that the first end is optically coupled to the plasma from outside the chamber. However, the optical fiber is positioned to avoid reflections from the semiconductor substrate. A processor is connected to the second end of the optical fiber for analyzing the plasma.
In a preferred embodiment, the chamber window has a substantially planar portion and a substantially tubular portion extending outwardly therefrom. The tubular window portion has a closed end opposite the planar window portion. The first end of the optical fiber is connected to the closed end of the tubular window portion. Advantageously, the tubular window portion is positioned at a predetermined angle relative to the planar window portion so as to be optically coupled to view the plasma, while avoiding reflections from the semiconductor substrate to thereby eliminate the interference. Any angle sufficient to avoid reflections from the wafer may be employed in the apparatus, with an angle of about 60xc2x0 from the planar window portion being a preferred angle.
A method aspect of the invention for semiconductor processing includes generating a plasma within a chamber having a window, connecting an optical fiber to the window so as to be optically coupled to the plasma while avoiding reflections from the semiconductor substrate, and processing signals from the optical fiber for analysis of the plasma.